Magnetic noise in the magnetosheath in the frequency range 3-300 hz

Abstract

Preliminary results are presented concerning rapid magnetic field variations (3–300 hz) observed in the magnetosheath by the OGO 1 search coil magnetometer spectrum analyzer. Broadband signals are continuously present in the magnetosheath at these frequencies that are relatively intense and highly variable on a time scale of tens of seconds. Power spectral density estimates for several representative intervals lead to This result extends previous estimates for the fluctuating magnetic fields in the magnetosheath up to 300 hz and reveals an apparent change in the frequency dependence of the spectrum somewhere between 0.1 and 1 hz. The observations are discussed in terms of the two transverse modes of plasma wave propagation that exist at frequencies below the electron gyrofrequency. Arguments are presented suggesting that some high-frequency bursts seen in interplanetary space just outside the bow shock are actually precursor waves propagating upstream against the solar wind.